The present invention relates generally to x-ray imaging systems. More particularly, the present invention relates to systems and methods of improving heat transfer and lowering peak temperatures within an x-ray imaging tube.
Traditional x-ray imaging systems include an x-ray source and a detector array. X-rays are generated by the x-ray source, pass through an object, and are detected by the detector array. Electrical signals generated by the detector array are conditioned to reconstruct an x-ray image of the object.
In general, an x-ray generating device is formed with a vacuum housing that encloses an anode assembly and a cathode assembly. The cathode assembly includes an electron emitting filament that is capable of emitting electrons. The anode assembly provides an anode target that is spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode. In operation, electrons emitted by the cathode filament are accelerated towards a focal spot on the anode target by placing a high voltage potential between the cathode and the anode target. These accelerating electrons impinge on the focal spot area of the anode target. The anode target is constructed of a high refractory metal so that when the electrons strike, at least a portion of the resultant kinetic energy generates x-radiation, or x-rays. The x-rays then pass through a window that is formed within a wall of the vacuum enclosure, and are collimated towards a target area, such as a patient. As is well known, the x-rays that pass through the target area can be detected and analyzed so as to be used in any one of a number of applications, such as a medical diagnostic examination.
In general, only a very small portion—approximately one percent in some cases—of an x-ray tube's input energy results in the production of x-rays. In fact, the majority of the input energy resulting from the high speed electron collisions at the target surface is converted into heat of extremely high temperatures. This excess heat is absorbed by the anode assembly and is conducted to other portions of the anode assembly and to the other components that are disposed within the vacuum housing. Over time, this heat can damage the anode, the anode assembly, and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Several approaches have been used to help alleviate problems arising from the presence of the high operating temperatures in the x-ray tube. For example, in some x-ray devices the x-ray target, or focal track, is positioned on an annular portion of a rotatable anode disk. The anode disk (also referred to as the rotary target or the rotary anode) is then mounted on a supporting shaft and rotor assembly that can then be rotated by some type of motor. During operation of the x-ray tube, the anode disk is rotated at high speeds, which causes a focal spot on the focal track to continuously rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with any given focal spot along the focal track for only short periods of time. This allows the remaining portion of the track to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat absorbed by the anode.
While the rotating nature of the anode reduces the amount of heat present at the focal spot on the focal track, a large amount of heat is still present within the anode, the anode drive assembly, and other components within the evacuated housing. This heat must be continuously removed to prevent damage to the tube (and any other adjacent electrical components) and to increase the x-ray tube's efficiency and overall service life.
Prior art x-ray generating devices have taken different approaches to removing this heat from the x-ray tube. In one approach, prior art x-ray generating devices rely upon the use of a second outer housing, separate from the housing forming the vacuum enclosure, to provide a variety of functions, including cooling of the x-ray tube with a coolant, and preventing excessive radiation emissions. This outer housing adds cost and complexity to the x-ray generating device, and can reduce its long term reliability.
In another approach, a single integral housing design is implemented and designed to cool the x-ray tube while also providing radiation shielding. Such a design relies on radiative heat transfer from the anode target to the housing. Various processes, such as sandblasting, grinding, and greening can be used to increase the emissivity of the frame for improved heat transfer, with sandblasting and grinding providing for an emissivity of about 0.35 for radiation having a wavelength of approximately 1.5 micrometers and greening providing for an emissivity of about 0.73 for radiation having a wavelength of approximately 1.5 micrometers. While such emissivity values may be adequate and provide enough cooling to operate the x-ray tube, such values do not provide for optimal performance of the x-ray tube. That is, performance of the x-ray tube could be improved by further increasing the emissivity value of components in the x-ray tube. The improved heat transfer would serve to decrease the maximum temperature imposed on critical components in the x-ray tube, thus leading to increased tube life and increased throughput/performance of the tube.
Another consideration for designs incorporating a single integral housing is that they provide adequate radiation shielding. In particular, such designs require the use of a layer of x-ray shielding material, such as lead, on the housing walls to prevent unwanted radiation emissions. This adds cost and manufacturing complexity to the device, increases its overall size, and may not be desirable from an environmental and safety standpoint.
Therefore, a need exists for an x-ray tube design that can provide improved cooling of the anode assembly and other components within the vacuum enclosure to provide for optimal performance of the x-ray tube. It would also be desirable for the x-ray tube to provide sufficient levels of radiation containment without using lead shields and the like.